Non-Dopaminergic Treatments for Motor Control in Parkinson's Disease: An Update.

Glutamatergic, noradrenergic, serotonergic, and cholinergic systems play a critical role in the basal ganglia circuitry. Targeting these non-dopaminergic receptors remains a focus of ongoing research to improve Parkinson's disease (PD) motor symptoms, without the potential side effects of dopamine replacement therapy. This review updates advancements in non-dopaminergic treatments for motor control in PD since 2013. To date, no non-dopaminergic selective drug has shown significant long-term efficacy as monotherapy in PD. The largest area of development in non-dopaminergic targets has been for motor complications of dopamine replacement therapy (motor fluctuations and dyskinesia). For treatment of motor fluctuations, safinamide, zonisamide, and istradefylline are currently approved, and novel glutamatergic and serotonergic drugs are in development. Long-acting formulations of amantadine are approved for treating dyskinesia. Several non-dopaminergic drugs have failed to show anti-dyskinetic efficacy, while some are still in development. Non-dopaminergic targets are also being pursued to treat specific motor symptoms of PD. For example, CX-8998 (a calcium channel modulator) is being evaluated for PD tremor and rivastigmine may improve gait dysfunction in PD. Drug repurposing continues to be a key strategy for non-dopaminergic targets in PD, but the field needs to increase discovery and availability of such drugs.